One-way clutches are used when it is desired to let a rotating member or race rotate freely in only one direction relative to another member, and are generally of the sprag element type or the roller type. In the sprag element type, both members or races present cylindrical surfaces to the sprag elements, but in the roller type either an inner or outer race has a series of cam ramps cut therein. A series of rollers, one for each cam ramp, are held in a cage which is tied to the cam race and a series of energizing springs mounted to the cage continually urge each roller into a ready position of touching engagement with one of the cam ramps on the cam race and with the other race, or ring, which is cylindrical.
In the case of the outer cam design, when the cam race is over-running the inner ring, the clutch will be "centrifugally disengaging" above a certain critical speed. That is, above the critical speed, the rollers will experience enough centrifugal force to overcome the spring force of the energizing spring and will move out of ready position up the ramp to become disengaged from the inner ring. In applications where it is important to have a quick lock-up, as in an automatic transmission stator, operation at speeds above critical speed is difficult. While inner cam designs are more centrifugally neutral, at certain high critical speeds of the over-running inner cam race relative to the outer ring the cam ramp may get sufficiently "ahead" of the roller that a similar disengagement and high speed instability of the roller occurs.
In some applications, such as the over-running clutch in a starting motor, the centrifugal disengagement is actually desirable to cut down on friction in the over-running mode. U.S. Pat. No. 2,044,197, to Barthel, shows an over-running clutch for such an application in which energizing springs bear not directly on the rollers, but upon pivoted blocks which bear on the rollers. In the over-running mode, centrifugal force pivots the blocks against the springs and allows the rollers to move from the cam ramps. This is desirable in the starter motor application, where quick lock-up is not necessary and where the clutch is usually in the over-running mode, as it reduces friction. A transmission presents a different problem, and must be able to provide quick lock-up response.